home → articles → Genetics & Performance | Email to a Friend

Target weights serve as useful tool in managing replacement heifers

published: October 31st 2008
by: Martha Hollida Garrett

By Martha Hollida Garrett
    Developing replacement heifers is somewhat like putting a jigsaw puzzle together, and managing those pieces is critical to ensuring the heifers have a long, productive life within your cowherds.  Target weight is a tool that breeders can utilize when developing heifers.
    Target weights are the body weight needed at specific times in the production cycle.  Examples of those events are weaning, first calving and first breeding seasons.  Numerous research trials investigating female reproduction and herd fertility have been conducted through the past several decades.  Review-ing these studies can be extremely beneficial when developing management plans for replacement heifers. Dr. Twig Marston, Kansas State University professor of animal science and industry, outlines target weights in the following chronological order.
    Weaning: Some will argue that target weight is least important at weaning, but Martson argues it is one of the most important target weights used by producers.  Many producers begin replacement heifer selection at weaning.  In the past many cattle herds were built selection programs around the heaviest heifers weaned to become their replacements.   Over time this has had a direct effect on other target weights established for the cowherd.  He said that during the past several years he has had discussions with cow/calf operators that are not keeping the big end of their heifer crops, but are keeping the middle cut--I know of no one that is intentionally keeping their lightest heifers for replacements.  By selling the big end and retaining the next cut, say selling the heaviest 25% of the heifers and retaining the next 15 to 25% of the crop for replacement purposes, heifers are being sold that cause an increase in mature cow weight and total operation feed costs as well as those heifers that will not maintain the performance of the herd.  So then the question becomes “what is the right weaning target weight for a replacement heifer?”  Practically that will probably be 50 to 100 pounds (more or less) above the group average.
    First breeding (start of the breeding season):  Typically producers should develop heifers to 60 to 65% of the mature weight at first breeding.  Recently, a University of Nebraska research report indicated that 50 to 55% was sufficient to assure fertility.  While this may be true,  heifers will need to be put on a sufficient nutrition program at some point early in their life to get them to catch up to their subsequent target weights.  Many trials have indicated that the light, low body condition cattle have difficulty birthing, insufficient colostrums production, lighter weaning weights, and lower subsequent pregnancy rates.  Therefore, it is safe to assume that properly developing heifers to 60 to 65% of the mature weight can have lifelong benefits.
    If a heifer calf has a 205-day weaning weight of 500 pounds and is expected to have a mature weight of 1,300 pounds, what are her target breeding weight and ideal average daily gain from weaning to breeding?  Thirteen hundred pounds times 60 to 65% is 780 to 845 pounds which equates to 280 to 345 pounds of post weaning weight gains.  If she is going to be bred at 14 months of age (420 days of age) then the feeding period is 420 minus 205 days (215).  The necessary average daily gain becomes the gain divided by the days, so in this example the heifer needs to gain 1.3 to 1.6 pounds per day. 
    Dr. Larry Olson, Clemson extension animal scientist, writes yearling replacement heifers cannot get bred early if they have not reached puberty prior to or early in the breeding season. Research has repeatedly shown age at puberty or first estrus in heifers is more dependent on weight than age. Furthermore, if 90% of a group of heifers are expected to be in heat during the breeding season, they should weigh a minimum of 65% of their expected mature weight at the beginning of the breeding season. This means the smallest heifer in the group not the average of the group should weigh 65% of her expected mature size at the start of the breeding season. The probability of getting heifers which have reached or exceeded their minimum target weights bred is 80-90%. In contrast, the odds of heifers lighter than their minimum target weight having reached puberty much less getting them bred is considerably lower and gets even lower the lighter they are.
    Body condition score can be used as a reference if the heifers have been developed properly.  Heifers that have achieved the proper target weight will be slightly above moderate body condition score.  On a scale of 1 (emaciated) to 9 (obese), the correct rank for replacement heifers should be in the 5-6 category. 
     In summary, heifer nutritional programs are an important segment of cowherd management.  Target weights can be used to determine if programs are on track and if adjustments are needed. 
    Olson points out that the minimum target weight for yearling heifers at the beginning of the breeding season differs widely both among and within breeds and crossbreeding combinations. Within a breed or crossbreeding combination there are very large differences in mature cow size from herd to herd due to herd differences in genetic selection for growth and level of available nutrition. The minimum target weights at the start of the breeding season for heifers of various expected mature weights are shown in the following table:
Expected Mature    Minimum Target
Weight                    Weight
900 lbs                    585 lbs
1000 lbs                  650 lbs
1100 lbs                  715 lbs
1200 lbs                  780 lbs
1300 lbs                 845 lbs
1400 lbs                 910 lbs
1500 lbs                 975 lbs
    These are minimum target weights for all heifers not average target weights for a group of heifers. Requiring yearling heifers to reach a minimum target breeding weight should result in at least 80-90% of heifers pregnant after 45-day breeding season.    Furthermore, it is strongly recommended that yearling heifers should also be bred to start calving at least 15 days ahead of the mature cow herd and for no more than a 60-day calving season. Calving 1st-calf, 2-year-old heifers early gives them more time to recover from  calving the first time.